1. Field of the Invention
The present invention relates to mine roof support systems, and more particularly relates to a mine roof support system comprising a sling that spans the width of the mine roof and is anchored into the rock formations above and behind each sidewall of a mine tunnel.
2. Description of the Prior Art
Sling support systems for underground mine tunnel roofs have been in existence for some time. Most of the older systems comprise two standard mine roof bolts anchored into the rock formation above the mine tunnel roof adjacent opposite mine tunnel walls at approximately 45.degree. from vertical. Each of these mine roof bolts passes through a connector of some sort that connects to a respective end of a bar or rod that spans the width of the mine tunnel roof. This horizontal rod may be formed in sections, if necessary. The horizontal rod is anchored to the mine roof bolts at each end thereof by a collar or sleeve that permits the horizontal rod to be tensioned as either mine roof bolt is further screwed into its own anchor imbedded in the rock formation above the mine tunnel roof and tunnel wall. This concept is basically shown in U.S. Pat. No. 3,509,726.
Subsequent modifications to this concept are shown in U.S. Pat. No. 4,679,967, which shows a sling bracket that is used at each end of the horizontal support bar. The sling bracket is anchored to the mine roof by a mine roof bolt, again anchored in the rock formation above the mine tunnel roof and tunnel wall. The horizontal span of rod attaches to the sling bracket in a manner to permit the horizontal rod to be tensioned independently of the two anchored mine roof bolts.
U.S. Pat. No. 4,946,315 shows an improvement on the previous design, that being the introduction of a third sling bracket at the approximate mid-point of the span of the horizontal rod, the third bracket being adapted to attach to a vertically oriented mine roof bolt for stabilizing the horizontal span to the rock formation directly above the mine roof.
U.S. Pat. No. 4,934,873 shows a variation on the tensioning of the horizontal sling. U.S. Pat. Nos. 5,193,940 and 5,238,329 both show mine roof sling systems that utilize a different threaded attachment mechanism for attaching the horizontal rod to the mine roof bolts that are anchored at the 45.degree. angle into the rock formation above the mine roof and mine sidewall.
U.S. Pat. No. 4,265,571 shows a mine roof sling system comprising a one-piece cable that is anchored at each end into the rock formation above the mine tunnel roof and the sidewall. This cable sling system includes an anchoring collar at each end of the cable that is driven into the bore hole and retained therein by a split sleeve anchoring tool, which remains in the bore hole to anchor the end of the cable therein. In addition, the cable anchor could comprise an expandable wedge-type anchor, and/or could also be anchored into the bore hole by cement.
Until the introduction of the cable sling, mine roof slings were constructed of separate horizontal sections (bars, rods, etc.) having plates or connectors at each end thereof that were somehow attached to mine roof bolts that were anchored into the rock formation above the mine roof, as previously described. In these cases, mine roof bolts were necessary because resin grout material was required to anchor the sling via the mine roof bolt into the rock formation. Because the resin grout material was necessary, bolts were required, as opposed to cables, because bolts could be rotated in the bore hole, and rotation of the mine roof bolt was necessary to thoroughly mix the resin grout material in order to effect a suitable anchor of the bolt in the rock formation. Although a single cable sling could be used, there was no way to rotate the ends of the cable as they were being inserted into their respective mine roof bore holes in order to mix the resin grout material. Therefore, the cable sling of U.S. Pat. No. 4,265,571 cannot use the stronger and preferable resin grout material, but rather must use cement, in combination with the friction shear resistance force between the bore hole and split sleeve anchor. The split sleeve anchors were required because cement alone (which did not require mixing) was insufficient to retain the cable in place. In addition, the split sleeve anchors required special air or hydraulic jacks and associated additional compressors, pumps, hoses, etc., for installation.